1984 300SD
I had the fuel pump rebuilt because performance was degrading.
Immediately, the car had a squeal accompanied by a slight loss of power
at cruising equilibrium (cruising without acceleration or deceleration).
I moved out of state a week later and could not take the car back to
that shop. I lived with the problem for months. I blamed this problem on
a sloppy rebuild job. Now I'm not so sure.

I just had to replace the ignition switch mechanism behind the key
tumbler because it locked up. This mechanism has a vacuum line to it.
The fuel pump squeal problem suddenly disappeared. I could park the car
for a week and I still had vacuum in the door locks. There was no leak
in that system. Perhaps the vacuum to the ignition switch is isolated
from door locks. This is a California car. There is some smog system
that I believe uses vacuum to modify transmission shift points and the
fuel pump is involved. There is a small black box on the side of the
fuel pump that I don't think is on a 49-states model. I'm happy to get
this sudden performance increase. I just can't understand why changing
the key switch solved this issue. Can anyone explain this mystery?